488 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



external ' envelopes,' parasitism, ' grazing ' by animals, and active 

 movement, as well as their various components. All of these can 

 be governed in turn by temperature, light, or the chemical composition 

 of the water as already mentioned. It should be emphasized, how- 

 ever, that temperature differences are less important than many 

 others, being often superseded by such factors as light, the effect of 

 which on photosynthesis in the twilight region (dysphotic zone) is 

 nevertheless in turn influenced by temperature. The long wave- 

 length ' red ' radiation is absorbed in the upper layers so that at a 

 depth of commonly 15-20 metres in clear water a vivid 'green' 

 coloration predominates where there are objects to reflect the rays. 

 Absorption and subsequent use for photosynthesis being largely 

 complementary to the colours of plants, the light of these and of 

 greater depths is utilized best by phytoplanktonic organisms that 

 are brownish (such as Diatoms) or reddish (such as the flagellate 

 Rhodomonas and certain Cyanophyceae). These brownish and reddish 

 types are often predominant in deep fresh waters, as are Brown and 

 particularly Red Algae in the sea. 



Owing apparently to wind and wave action as well as to the 

 mysterious avoidance of shallow water by Entomostraca, the character 

 of the general plankton is often peculiar along lake-shores, where the 

 abundance of phytoplankton may actually be greater than elsewhere 

 owing to turbulence and the low incidence of predation. On the 

 other hand, in deep waters where oxygen tends to be in short supply, 

 the temperature is often so low that few organisms can exist, and, 

 therefore, much less of this gas than usual is required for respiration. 

 Finally, in situations where oxygen is definitely deficient, anaerobic 

 (that is, living in the absence of oxygen) Bacteria often abound, in- 

 cluding many having a planktonic habit, while below, about the 

 surface of the bottom deposits, numbers of Bacteria computed to be 

 of the order of 100,000 or more per cubic centimetre are frequently 

 found. As well as by such deep-down productivity, the total biomass 

 may also be increased as a result of judicious fertilization (addition 

 of needed materials), an example being the supplying of phosphate 

 to oligotrophic lakes or ponds in which development is not limited 

 by poverty in nitrogen. 



In contrast to lakes, running waters are characterized by a true 

 potamoplankton only in those that flow far and gently. Even here 

 the turbulence is not sufficiently reduced to permit much strati- 

 fication, conditions being much more homogeneous than in lakes. 

 Another difference is the transport of water-masses frequently over 



